Compounds that can link together a thiol-bearing moiety and an amine-bearing moiety have found use in many diverse applications such as the linkage of cytotoxic agents, fluorophores or metal chelating agents to cell binding agents such as antibodies, growth factors or vitamins, or to large complexes such as liposomes (see U.S. Pat. Nos. 5,208,020; 5,416,064; 5,846,545; 6,340,701; 6,716,821; 7,217,819; 7,276,497; 7,388,026; 7,598,290 and U.S. Patent Publication No. 20100203007). Examples of such linkers include SPP, SPDP, SPDB and SMCC (FIG. 1) (see U.S. Pat. No. 4,563,304; 6,913,748; 7,276,497; Widdison et al., 2006, J. Med. Chem., 49, 4392-4408; Phillips et al., 2008 Cancer Res., 68, 9280-9290) Often one or both of the entities that are to be linked together have only limited solubility in water which results in poor aqueous solubility of the resulting conjugate.
Recently, it has been shown that antibody-drug conjugates, wherein the antibody and the cytotoxic drug were linked via charged crosslinkers, especially those that comprise a sulfonic acid substituent showed several benefits over the corresponding conjugates prepared with non-charged linkers (FIG. 2). These benefits include a) greater aqueous solubility of the antibody-drug conjugate, b) ability to increase the drug load on the antibody, while maintaining solubility and monomer content and c) greater therapeutic activity, especially toward multidrug resistant cells (see U.S. Patent Publication No. 20090274713 & 20100129314). Improved pre-clinical efficacy of antibody-drug conjugates prepared with the sulfo-SPDB crosslinker [FIG. 2, compound (V)] that comprises a sulfonic acid substituent, has also been recently reported (Kovtun et al., 2010 European J. Cancer, Suppl, 8, p 76, Abstract #235). The only previously reported synthesis of sulfo-SPDB is a lengthy and inefficient process comprising of 6 chemical steps (FIG. 3, see U.S. Patent Publication No. 20090274713). Thus there is a need to provide a simplified and scalable process for the preparation of charged crosslinkers, such as sulfo-SPDB, that comprise a sulfonic acid group. These and other needs are addressed in the current invention.